Bone marrow transplantation is becoming widely used in the treatment of acute leukemia, aplastic anemia, and selected solid tumors. Patients receiving blood product transfusions prior to transplantation have a higher risk of marrow graft rejection. Sind all present blood cell component therapy contains significant numbers of erythrocytes, leukocytes and platelets, the exact role that each cell population plays in inducing a sensitized state becomes important. Purified platelets are thought to be poorly immunogenic with reference to the major histocompatibility comples (MHC) antigens on their surace. In the murine model, immunization with purified platelets does not induce a significant primary antibody or cellular immune response. However, the use of purified platelet transfusions prior to bone marrow transplatation in the mouse does lead to rapid marrow graft rejection. Thie difference suggests that the mechanism of hematopoietic stem cell graft rejection is different from the basic mechanisms of antibody production or solid tissue allograft rejection. This project will study the hypothesis that: 1) Purified platelet immunization induces marrow graft rejection by a mechanism different from that involved in traditional allosensitization. 2) This platelet induced marrow rejection is genetically restriced to a region(s) of the MHC. 3) The understanding of the immune mechanism involved in marrow allograft rejection will allow newer approached to the control of graft function in clinical situations. The objectives of this project are to: 1) Develop a murine model to study the effect of highly purified platelet transfusions on bone marrow graft function. 2) Evaluate the role that the MHC plays in rejection of marrow grafts. 3) Determine the immunological mechanism(s) involved in platelet sensitized bone marrow rejection.